1. Field of the Invention
This invention relates to an edge-lit transflective non-emissive display. More particularly, the invention relates to non-emissive displays, such as electrochromic, ferroelectric, ferromagnetic, electromagnetic, and liquid crystal displays, where the external light source for the display is ambient light and/or artificial light generated at the edge of a flat light conducting pipe behind a non-emissive layer.
2. Description of Prior Art
A liquid crystal layer between transparent plates is probably the most common non-emissive light device; such a liquid crystal device is non-emissive in that the liquid crystal layer generates no light. Non-emissive displays may modulate light by absorption, scattering, refraction, or polarization changes. In a liquid crystal display, the liquid crystal acts as a shutter, or selective light conductive device. The display depends upon light being passed, or not passed, through the liquid crystal layer to generate the display. In other words, the liquid crystal layer must have an external source of light for the liquid crystal display to produce an image.
In a reflective liquid crystal display (LCD), the source of light for the display is ambient light which enters through the front of the display, passes through the liquid crystal layer (LCL), and reflects back through the LCL from a reflective, or mirror, surface behind the LCL. U.S. Pat. No. 4,519,678 issued to Y. Komatsubara et al is an example of a reflective LCD.
In a transmissive LCD, artificial light is generated behind the LCL, or at the edge of a flat light pipe which distributes the light uniformly behind the LCL. Examples of transmissive LCD's are taught in the article entitled "New Backlighting Technologies for LCD's" by K. J. Hathaway, J. A. Hawthorne and A. A. Fleischer in the SID (Society for Information Display) 91 Digest, pp. 751-754. This article discusses backlighting with flat fluorescent lamps. Such lighting is not attractive for LCD's because of problems with uniformity of light from the tubes, and because the tubes are relatively bulky and require too much electrical power for the typical LCD environment. Hathaway et al suggest that an edge-lit wedge-shaped light pipe behind the liquid crystal is the better approach to backlighting the LCD. The rear surface of the light pipe has a diffuser, or prismatic specular reflector, to shape the light distribution as a function of viewing angle.
Another example of edge-lit light piping for backlighting an LCD is taught in U.S. Pat. No. 4,616,295 issued to M. B. Jewell et al and entitled "Apparatus for Mounting and Illuminating Liquid Crystal Displays." A feature in the Jewell et al patent is varying the light diffusion pattern with the distance from the edge light to provide more uniform backlight for the liquid crystal. Many edge-lit light pipes for LCD's commercially available today use a pattern of diffusing dots that increase in frequency and/or size with distance from the edge light in order to produce more uniform backlighting of the LCD.
Transflective LCD's use an edge-lit light pipe behind the LCL and, in addition, the front surface of the light pipe is partially reflective so that ambient light is partially reflected back through the LCL. Transflective LCD's are shown in U.S. Pat. No. 3,994,564 issued to B. Somogyi, and U.S. Pat. No. 4,017,155 issued to K. Yagi et al. Transflective liquid crystal displays have the problem of optimizing the transflective optical element for both ambient light and edge light. Typically, the transflective element uses an edge-lit flat light pipe, and coats the front surface of light pipe with a 70/30 reflective material. In this design, 70% of the ambient light reaching the reflective material is reflected back through the liquid crystal layer. Conversely, only 30% of the light from the edge-lit light pipe is passed by the 70/30 coating through the liquid crystal layer. Accordingly, while transflective LCD's are operative with, or without, ambient light, they are not very efficient in power consumption when the edge light is being used.